1. Field of the Invention
The present invention relates to a conductive roll for developing, charging, destaticizing and transferring images to be used in an electrophotographic recording device. More particularly, it relates to a conductive roll of rubber or foamed material and comprises a core made of a highly conductive metal shaft and a coated film layer formed on the outer periphery thereof for controlling the resistance, and a method for manufacturing the same.
Electrophotographic recording devices widely used in copying machines and laser beam printers generally comprise a photosensitive member, on which static latent images are formed by charging and exposing the photosensitive member. A toner is used to develop a latent image on the photosensitive member by causing the toner to be absorbed. The toner adhered on the image is then transferred onto a sheet of transfer paper. By destaticizing the photosensitive member to a predetermined level of potential and by wiping out the residual toner thereon, the photosensitive member is ready for subsequent recording. The toner transferred to and carried by the paper is eventually fixed thereon as it becomes fused and pressed against the paper, whereby recording of images on the paper is completed.
As a means to charge the photosensitive member of the electrophotographic recording device to a predetermined level of potential, to apply a predetermined level of potential to the transfer paper conveyed into the transfer zone for image transfer, or to uniformly charge to a predetermined level the charged zone of the photosensitive member after image transfer, corona discharge means comprising a thin wire to which is applied a high voltage of several hundreds to several thousands of vol tage has been widely utilized.
This type of corona discharge means is defective in that active molecules such as ozone generated during corona discharged may deteriorate the photosensitive member and other parts or adversely affect the human body. Use of high voltage also entails the danger of electrification as well as the problems of maintenance and servicing of the damaged or broken wire.
To overcome these problems, a contact charge means of a roller type (hereinafter referred to as a conductive roll) has been proposed which made of a conductive rubber roller and is directly contacted with the photosensitive member to apply predetermined level of voltage. The roll offers excellent features such as that it does not require high voltage as in the corona discharge means and ozone generation is almost negligible.
An important aspect of the conductive rollis that a highly close contact between the roll and the photosensitive member is essential in order to charge the latter uniformly at a given potential.
Development of an effective means for that purpose has been awaited.
To improve the close contact, a softening agent in the form of a liquid substance of a low moleculer weight such as oil is mixed the starting material for the roll. However, the softening agent would migrate onto the rolls surface and spoil the photosensitive member.
To make the roll electroconductive, an electroconductive carbon powder such as Ketjenblack EC and acetylene black is added. However, even slight changes or irregularity in the powder addition may vary the electric resistance greatly.
The conventional conductive roll is further defective in that it is difficult to control and regulate the electric resistance. It is also difficult to give the roll the required dielectric strength characteristics.
The electric resistance of the conductive roll is susceptible to changes in environmental conditions such as humidity and temperature, making it difficult to charge the photosensitive member at a constant level of potential.
The conventional conductive roll can be very brittle and susceptible to cracks and wear depending on the material. Development of conductive roll with little chronological changes has also been awaited.
To manufacture a conductive roll, a roller 100 made of rubber or foamed urethane is coated with a liquid coating material by a wet or dry process such as electrostatic coating, dipping or rolling as shown in FIG. 1. After drying, a coated film layer 101 for controlling the resistance is formed thereon (FIG. 2). In order (1) to prevent the coating material from adhering to the shaft 102, and (2) to form the end portion of the coated film layer 101 as shown in FIG. 2, it is the general practice to mask the shaft 102 with a tube 103 whose outer diameter a is the same as that of said end portion and whose inner diameter is substantially the same as the outer diameter bof the shaft 102 (FIG. 3). After applying the coating material with the shaft 102 being thus masked and drying the coating material, or alternatively after coating by a dry process, the coated film layer 101 as shown in FIG. 4 is cut at the portion c and the masking tube 103 is removed to obtain a roll as shown in FIG. 2.
To prevent sparking between the photosensitive drum and the roller made of rubber or foamed material having a very low electric resistance, it is necessary for the coated film layer to project beyond the roll end by a considerable length. But if the extended portion is too long, the film layer may come in contact with the bearing and become torn.
If there exists air between the tube and the roll during extrusion of the film layer, the air may collect and produce a swell, deteriorating the dielectric strength of that particular portion. Moreover, a trace of air present between the masking tube 108 and the shaft 102 or between the tube 108 and the end portion of the roller 100 is likely to permeate into the film layer at the end of the roller 100 and form pinholes 101a (FIG. 5), making the film layer 101 susceptible to cut (FIG. 6).
When there are pinholes 101a or if the edge of the film later 101 becomes flush with the end face of the roller 100 as the film is cut, spark discharge is likely to occur at this portion of the roller 100, damaging the photosensitive drum.
The present invention was contrived to overcome these defects encountered in the prior art, and aims at providing an electroconductive roll which contacts closely with the photosensi live member, is least likely to contaminate the photosensitive member, minimizes fluctuations of electric resistance caused by environmental changes, and is less likely to become cracked or worn out for an extended period of time. The present invention also aims at providing a method for manufacturing such conductive rolls.